1. Field of the Invention
The present invention relates to an inkjet printing head and a method of manufacturing the inkjet printing head. In particular, the present invention relates to an inkjet printing head using a method in which a state of liquid is changed along with a rapid volume change of the liquid (generation of a bubble) by applying energy such as thermal energy to the liquid, and in which an acting force caused by this change of the state allows ink to be ejected from an ejection opening; and to a method of manufacturing the inkjet printing head.
2. Description of the Related Art
In general, in an inkjet printing head of this type, provided is a liquid path extending towards an ejection opening from an upstream side in an ink supplying direction, and the liquid path is provided with a heater (such as an electrothermal transducer) which generates thermal energy applied to ink. Then, a state of ink on the heater facing the liquid path is changed (film boiling generates a bubble) when the heater is driven, and this change causes a pressure with which ink existing on an ejection opening side from the heater to be ejected. This bubble phenomenon itself, however, does not have any directional characteristics, so that the pressure caused by the bubble affects, in the ink channel, not only on the direction in which the ink is to be ejected, but also on the upstream side in the ink supplying direction. This phenomenon generates energy loss, thereby reduces the amount of the energy which is to effectively contribute to ink ejection, decreases an ink ejection speed, and thus deteriorates printing quality. In addition to the above problems, the pressure towards the upstream side in the ink supply direction causes a delay in an operation to replenish (refill) ink of the amount equivalent to that lost due to the ejection. Thus, the pressure is also a factor to prevent printing speed from being speeded up.
In recent years, a demand for printing an image with a stable printing quality at high speed has been increasing. This demand is particularly apparent for printing apparatuses for industrial applications. Thus, in some cases, employed is the following configuration which aims to increase effective use of energy, and to facilitate a smoother refill operation. In this configuration, a movable member is provided in the liquid path, and the movable member operates as a valve in response to the generation of bubble. Thus, the movable member controls the growth of bubble so that the bubble would not go to the upstream side in the ink supply direction.
In Japanese Patent Application Laid-Open No. 63-197652 (1988) (referred to as JP 63-197652 A, below), disclosed is a configuration in which a valve is integrally formed as a single body by utilizing part of a substrate having a heater formed thereon. JP 63-197652 A also cites another configuration as a conventional example. In this configuration, a printing head includes a structure in which a valve or a member (a valve retaining member) having the valve formed thereon is separated from a member (substrate) having a heater formed thereon, and the valve or the valve retaining member is attached to the substrate in a process later performed. Then, in terms of this configuration, the following problems are described: since it is difficult to form a fine valve, it is not easy to form a fine liquid path; and since the valve is attached to the liquid path by use of an adhesion method or the like, not only does the number of assembly processes increase, thereby leading to an increase in manufacturing costs, but also a reduction in reliability, and instability of ejection performance may be caused depending on the attachment accuracy. Thus, JP 63-197652 A discloses the structure in which the substrate and the valve are previously integrated into a single body, and a method of manufacturing the same in order to solve these problems.
In contrast to this, in FIG. 11 of Japanese Patent Application Laid-Open No. 10-16243 (1998) (referred to as JP 10-16243 A, below), disclosed is a method of manufacturing a printing head having a two-body structure formed of a substrate and a valve retaining member. Specifically, JP 10-16243 A discloses the method in which a supporting member at least having its surface formed of metal is disposed on a substrate including a resistor element (a heater), and in which a metallic separation wall (a valve retaining member) having a movable member is fixed to the substrate by the supporting member. In the embodiment of the invention, a method of bonding and fixing, that is, attaching, the substrate and the separation wall to each other as follows. In this method, supporting members each formed of Au or the like of a stud bump type are respectively embedded into two substantially rear portions of the substrate; a separation wall formed of Ni or the like is positioned and mounted on the supporting members in the two portions; and a metal alloy layer between the supporting member and the separation wall is formed by performing a heat treatment or the like on the supporting member from above the separation wall. In this way, the substrate and the separation wall are bonded and fixed to each other. Furthermore, JP 10-16243 A describes the following effect of this method. To be more precise, although the hardening and shrinkage of an adhesive agent adversely affects the attachment accuracy in a case where the adhesive agent is used, this method makes it possible to prevent this disadvantage from occurring.
Both JP 63-197652 A, and JP 10-16243 A intend to achieve an efficient use of energy for ink ejection, and a smoother refill operation, but employ the different basic configurations of the printing heads for achieving these purposes. Specifically, while the printing head disclosed in JP 63-197652 A employs the structure in which the substrate and the valve retaining member are integrally formed in advance (hereinafter, termed as a single-body structure), the printing head disclosed in JP 10-16243 A employs the structure in which the substrate and the valve retaining member each being formed as a separate member are adhered to each other (hereinafter, termed as a two-body structure).
As a result of a dedicated examination made by the inventors of the present invention on these structures, the inventors have obtained the following findings.
Specifically, first, in JP 63-197652 A, in a process of manufacturing the member of the aforementioned single body structure, a layer made of a predetermined material (polysilicon in JP 63-197652 A) is formed on a substrate, and then a portion which is to become a valve is superposed on the layer. Thereafter, the layer existing on a liquid path portion below the valve is etched. Thus, since the etching needs to be carried out in an area below the valve from the periphery of the valve, it is necessary to have some space in the periphery portion of the valve except a portion of the valve to be supported in a cantilevered manner, in order to allow the etching process to be carried out. Specifically, dimensions or a shape of the valve is limited when the etching process is taken into consideration. For this reason, desired dimensions of the valve, that is, a project area of the valve to the heater becomes small. Thus, there is a concern that the effective use of ejection energy, and a smoother refill operation, which are the desired objects, may not be achieved to a sufficient extent. This concern may particularly become a problem in a case where used is a printing head or a printing apparatus for industrial applications of which a stable ejection operation at high speed is strictly required.
Accordingly, it is strongly preferred that a valve having dimensions or a shape which is suitable for the desired objects be formed. Thus, it is advantageous to employ the two-body structure from a viewpoint of design and manufacturing. In this case, however, the problems recognized in JP 63-197652 A, that is, the reduction in reliability, the instability of ejection performance, and the like due to a decrease in the attachment accuracy of the valve retaining member need to be solved appropriately, as a matter of course.
Here, the attachment accuracy of the valve retaining member needs to be secured by attaching the valve retaining member without having undesirable warpage or lifting with respect to the substrate. However, in a case where the valve retaining member and the substrate are caused to adhere to each other in processes of applying a liquid adhesive agent, and then of curing the adhesive agent by heating, various problems to be described below occur.
Firstly, when the number of points where the adhesive agent is to be applied is small, the undesired warpage or lifting cannot be suppressed effectively. Moreover, since the adhesion strength between the substrate and the supporting member is weak, there is a concern that the valve retaining member may be separated from the substrate due to the flow of ink.
In a case where the number of the application points and the amount of the adhesive agent to be applied are increased in order to solve these problems, the adhesive agent flows from the application point to the periphery thereof, and therefore the drops of the adhesive agent on the neighboring application points are connected to each other. This is because the adhesive agent is in a liquid state when being applied thereto. As a result of this, the valve retaining member and the substrate become in a state where they adhered to each other in a contiguous wide area (all over the surface in the extreme case) with the adhesive agent. In this case, a large amount of stress is generated on the adhesion interface by heating in the process of curing the adhesive agent, or by thermal influence occurring along with a printing operation. Specifically, the stress is generated in the adhesion portion due to curing and shrinkage of the adhesive agent, or a difference between the linear expansion coefficients of the adhesive agent and the substrate. This stress may generate a fine crack in the substrate. In general, the substrate is provided with a wiring of aluminum or the like for selectively driving the heater, so that an electrical short may occur when ink flows into the crack which has been generated. Moreover, an excessive amount of the adhesive agent applied thereto may inhibit ink from flowing in the printing head or the liquid path.
Furthermore, a metering discharge device (a dispenser) is used in general for the purpose of applying a predetermined amount of an adhesive agent to a desired position, but a shape of the applied adhesive agent cannot be accurately controlled by use of the metering discharge device. Accordingly, this produces a difference among the shapes of the drops of the adhesive agent after cured at the respective applied positions, and thereby generates a variation in the fixation state of the valve retaining member. Thus, it becomes extremely difficult to maintain a stable adhesion state, that is, the stable attachment accuracy.
In contrast to this, the method disclosed in JP 10-16243 A employs a structure in which, with application of a bonding technique, the valve retaining member is joined to the substrate with the supporting members provided on the two rear portions on the substrate. In such a configuration, problems related to the curing of the adhesive agent or the like do not occur. As in the case of using an adhesive agent, however, the warpage or lifting may not be suppressed effectively. Thus, there is a concern that the valve retaining member may be separated from the substrate.